A rotary drive of the type concerned is known from EP 0 628 454 A1. In this rotary drive a driving gear configured as a ring gear is coupled via three planetary gears to a sun gear forming a pinion, the planetary gears engaging in the internal toothing of the ring gear and in the external toothing of the sun gear. Formed at the outer periphery of the ring gear is a rotary piston extension which forms, together with the ring gear, a single-vane rotary piston. This single-vane rotary piston moves in an annular space which surrounds the ring gear and may be acted upon with pressurized gas.
The invention, in contrast, provides a rotary drive for a belt tensioner which is distinguished by a particularly compact construction.